A tape drive has been known, which records and reproduces information by taking out a magnetic tape being a recoding medium from a tape cartridge having therein a single reel around which the magnetic tape is wound. This kind of tape drive is widely used for backing up data for a system such as a computer server dealing with a huge amount of data.
Although there are various kinds of tape cartridges that have a single reel therein as described above, for example, there has been known the one in which a leader block connected to one end of the magnetic tape is disposed in a housing of the tape cartridge.
When the tape cartridge having the leader block is loaded in the tape drive, the leader block is chucked by a chucking mechanism, taken out by a transport mechanism, and conveyed to a take-up reel of the tape drive. The magnetic tape taken out from the tape cartridge is taken up by a take-up reel of the tape drive, and, simultaneously, the information is recorded on or reproduced from the magnetic tape by the magnetic head of the tape drive.
The tape reel to which the magnetic tape is wound conventionally comprises an upper flange unit and a lower flange unit which are disc-like and for guiding both sides of the magnetic tape, and a cylinder-like drum unit and to which the magnetic tape is wound. The drum unit is formed, for example, integrally with the lower flange unit.
FIG. 20 is a bottom view of a conventional tape reel 50 viewed from the side of a lower flange unit 52 thereof. A circular projection 52f is formed at a center portion of an under surface of the tape reel 50. The projection 52f has an outer peripheral portion 52r which engages with a circular opening formed on a lower shell of the tape cartridge. The outer peripheral portion 52r has at an inner portion thereof a ring-shaped chucking gear 52b engaged with a gear formed on the top of a rotary shaft of the tape drive side.
Between the outer peripheral portion 52r and the chucking gear 52b, three reference planes 52t supported by a reel support on the rotary shaft are provided on the same circumference at even angle intervals. Portions other than the reference planes 52t are to be voids 52p. 
In the figure, a reference numeral 52a depicts a rotation regulation gear formed on a periphery of the lower flange unit 52.
As in the conventional example described above, there is, as a related art document, for example, the following patent document 1, which discloses a structure of a tape reel in which the reference plane 52t and the outer peripheral portion 52r are continuously formed.
Patent Document 1
Japanese Patent Application Publication Hei 10-241324 (FIG. 1)
By the way, such a tape cartridge as described above is mainly used when bulky and important data is stored and it is required to have high reliability in various viewpoints.
For example, in order to accelerate a series of operation in the tape drive, there are demands for operations at higher speed in taking out the leader block, forwarding the magnetic tape and rewinding the magnetic tape. On the other hand, at the same time, the tape cartridge is required to be tough so that breaking of the magnetic tape and damage on the tape edge are hardly occurs and to have a high reliability in recording/reproducing operation.
With regard to the tape reel which the magnetic tape is wound around, the tape reel is usually formed with an injection-molded body of a synthetic resin material. The mold quality largely affects on runnability of the magnetic tape and the reliability in recording/reproducing operation.
In particular, since the projection portion 52f of the lower flange unit 52, on which the chucking gear 52b, the reference plane 52t or the like are formed, has an extremely complicated shape, irregularity of a sink amount of the resin after molding on the lower flange unit 52 largely affect on the edge of the magnetic tape which is wound around the drum unit, which becomes a cause of deterioration in straight runnability of the tape at high speed and reliability in recording/reproducing operation.
Here, FIG. 21A is a view showing the projection portion 52f of the lower flange unit 52 in the conventional tape reel 50, and FIG. 21B is a schematic view showing a flow of the resin at a portion shown by a cross-sectional line SS1 in the reference plane 52t (FIG. 21A) at the time of molding.
As shown in FIG. 21A, three gate positions G1 are all provided in the voids 52p. Since the portions constituting the reference plane 52t are thicker and bulky than the other portions and, further, they are away from the gate positions, injection pressure is hardly applied thereto. Furthermore, the portion where the reference plane 52t is formed is a portion where a resin flown out from the other gate merges.
Accordingly, the resin is easily short at the portions constituting the reference plane 52t and a sink F1 easily occurs on the opposite side (a plane on which the tape is wound) of the reference plane 52t after hardening the resin, as shown in FIG. 21B.
Irregularity of the sink F1 largely relates to surface wobbling of the lower flange unit 52. The surface wobbling disturbs the straight runnability of the magnetic tape. The contact between the edge of the magnetic tape and the flange unit damages the edge of the magnetic tape. Thus, there occurs a problem that reliability in recording/reproducing operation of the data is deteriorated.
The present invention was made in view of the above-mentioned problem, and it is an object of the present invention to provide a tape reel in which a flange unit thereof is improved in its molding quality and whose reliability at the time when a magnetic tape is running at high speed is improved, and a tape cartridge having the tape reel.